Process for making perchlorates



June 1950 J. c. SCHUMACHER 2,512,973

PROCESS FOR MAKING PERCHLORATES Filed Oct. 31, 1945 3 Sheets-Sheet 2 INVENTOR Jose; C JC/IUM/ICf/ER,

ATTORNEY June 1950 J. c. SCHUMACHER 2,512,973

PROCESS FOR MAKING PERCHLORATES Filed Oct. 31, 1945 I5 Sheets-Sheet 3 Na Cr O NaCJIO NaOH Steam HfO M101 A Dissolver B Gases Elecfrolyhc Cell C Storage Tank I r Auxiliary D Electrolytic Cell N GO Storage E G 7 a2 3 Tank W1 H O K01 l F l i Percipiraior K01 Dissolver l a I G Thlckener- H -Separator F EL Recovery i K Refrigeration Crysfallizer N NGCIQ Recovery Centrlfuge NaC1O or N001 Waste Crysltals M KC|O Dryer INVENTOR JOSEPH c. SCHUMACHER K010 Crystals aw/KM ATTORNEY Patented June 27, 195G UNITED STATES PATENT OFFICE PROCESS FOR MAKIN G PERCHLORATES Application October 31, 1945, Serial No. 625,857

6 Claims.

This invention relates to a process for making perchlorates and particularly perchlorates of the alkali metals.

One object of the invention is to provide an improved process for making alkali metal perchlorates. Another object is to provide an economical method for making potassium perchlorate by the electrolysis of sodium chlorate. A further object is to provide means for separating potassium perchlorate in a pure state from the electrolyte of a perchlorate electrochemical process. Still another object is to provide a two stage electrochemical process for substantially completely converting sodium chlorate to sodium perchlorate.

In generalmy process consistsin the electrolysis of a Substantially saturated aqueous solution of sodium chlorate in the presence of sodium chromate and magnesium chloride as catalysts, and with a controlled pH value in the electrolyte. From the electrolyte of an electrolytic cell, the perchlorate is separated as potassium perchlorate by the addition of potassium chloride, the potassium perchlorate being much less. soluble in water than the sodium salt. I have'found that by starting with a high concentration of sodium chlorate in the electrolyte and recirculating the electrolyte through an electrolytic cell repeatedly, that the conversion 01' the chlorate to the perchlorate may be efiected to the extent of about 90%, thus yielding an effluent electrolyte of high concentration insodium perchlorate. I have also found that much greater efiiciency may be attained by again electrolyzing the effluent electrolyte from the above mentioned first electrolytic cell in an auxiliary electrolytic cell where the electrolyte is. recirculated in and out of the cell until only about one or two percent of the sodium chlorate content remains, the balance having been oxidized up to the sodium perchlorate compound. I havealso found that the addition of magnesium chloride, along with the sodium chromate, as a catalyst, greatly assists in preventing undesirable side reactions. It is believed that this is brought about by the formation of a coating on the cathode surfaces which enables the hydrogen to be discharged as molecular hydrogen before effecting unwanted chemical reduction of the chlorate and perchlorates. The amounts of the sodium chromate and-magnesium chloride may be Varied, but it is desirable to keep. them to as low an effective content as possible because of the additional cost of completely removing them from the mother liquor in the steps subsequent to the electrolysis. A minimum of three grams per liter of sodium chromate and, one gram per liter of magnesium chloride has been found to be effective, while amounts of sodium chromate up to 15 grams and of magnesium chloride up to 10 grams per liter have been successfully used, so far as the catalysis of the electrochemical reactions is con.- cerned.

The first mentioned electrolytic cell is operated with a continuous feed into the cell and the continuous withdrawal of a proportion of the electrolyte, which is. then accumulated to be used in charging the auxiliary cell, which, is operated as a batch process in which the electrolyte. is recirculated in and out of the cell until the desired conversion of the chlorate. to, perchlorate has been attained. ,The electrolyte oivthe auxiliary cell is then discharged completely, and the, cell refilled with a new charge consistin of the effluent electrolyte withdrawn from the. first m ntioned continuous cell which has been accumulated in an intermediary storage, tank.

I have further found thatthe electrochemical change from the chlorate to the perchloratelis facilitated by maintenance in the electrolyte of a pH value of 6.0 to 6.8, a temperature of about 55 C., and the presence therein as catalyst of small amounts of sodium, chromate and of magnesium chloride.

My process will be understood by reference to the drawing Figure '7 which shows diagrammati cally the flow of materials. The dissolver A is used to form an influent electrolyte by dissolving the proper proportions oiv solid sodium chlorate, sodium dichromate, magnesium chloride. and sodium hydroxide in water, steam being con.- veniently introduced to assist in the solution of the salts and to heat the solution to a temperature. of about 55 C. The influent electrolyte from the dissolver in a typical example contains about 500 grams of sodium chlorate together with about 5 grams sodium chromate and 2 grams magnesium chloride per liter, and with enough sodium hydroxide to give a pH. of 6.0 to 6.8. It has been found that if the influent electrolyte has a pH vall fi in the, range of 6.0 to 6.8, that it remains at about this. acidity throughout the cycle, of electrolytic reactions, but if the influentelectrolyte starts at a pH value above this, for. example 6.9, the pH value tends to rise to '7 and above during the electrolysis, withw lowered eificiency. This solution is passed in a continuous stream into the first electrolytic cell B which preferably consists of an elongated steel trough with arrangements for water cooling both on the outside and by pipe coils intermediate the anodes, the

pipe coils and the side walls of the cell servin as cathodes. Such an electrolytic cell is described in my co-pending patent application Serial No. 625,859 filed October 31, 1945, now

Patent No. 2,475,147, issued July 5, 1949, and shown in Figures 1 to 6 inclusive.

Figure 1 is a top elevational view of a preferred form of my electrolytic cell;

Fig. 2 is a side elevational View of the same with parts broken away to show the interior ar 1 rangement;

Fig. 3 is an end elevational view of the same; Fig. 4 is a cross-sectional view taken on the line 4-4 of Fig. 2;

Fig. 5 is a cross-sectional view taken, on the,

on the outer side walls of the tank, the steel tank and attached'steel parts inside the cell constitut-- ing the cathode.

Referring-to the drawings, my electrolytic cell consists, of an elongated steel tank I! with an open top and outwardly extending flanges [2 around the upperedge.

jackets l3 welded to the two side walls HA of the tank I I, these being subdividedby horizontal partitions l4 to distribute the circulating cooling liquid. Further control of thetemperature in the cell is attained by the use ofa tier of steel tubes l5 extending centrally through the electrolyte compartment of the cell between the anodes H, the ends of these tubes being connected to manifolds 16 on the two ends of the tank H so that the cooling liquid. may be circulated not only through the side wall jackets l3 but also through these tubes disposed between the platinum anode l1 within the electrolyte compartment. ofthe cell. The cell cover plates l8 consist of sheets of chemically resistant artificial stone material. for example, sheets made of compressed mixtures of Portland cement and asbestos which sheets are attached by bolting to'the flange l2 around the upper edge of the tank. Slit openings lS'fitting the platinum electrode sheets 2!! are provided in this artificial, stone cover l8 for the insertion of,

the anodes. The steel surfaces of the tank H and thewater-cooling tubes l5 in-contact with the electrolyte serve as .thecathode for the cell.

' The electrolyte is introduced through the influentQpipe Zia and is withdrawn through the overflow or efiluent pipe ZZatube. The cell is especially adapted for use in a process requiring the circulation'or recirculation of the electrolyte in and out of the cell but it may also be used in a discontinuous processor in a batch vor int rmittent process in which the cell is filled with electrolyte which remains in it until the electrochemical reaction is completed to the desired stage.

In most electrochemical processes of the type to which this cell is adapted, there is evolved a mixture of gases oftenin explosive proportions.

In the case of the electrochemical oxidation of sodium chloratetosodium perchlorate forex ample, varying amounts and proportion of hy- Channels -for cooling liquid are provided by longitudinally placed steel quantity of the metal as possible.

drogen, oxygen, and chlorine gases are evolved, and it is necessary, in order'to avoid explosions, to remove this gas or dilute it with another gas to a mixture which is no longer explosive. Provision is made in my cell for the introduction of a sweeping out gas through th influent pipe 23 and the removal of the mixture of gases from the efiiuent pipe 24.

In electrochemical reactions requiring the use of platinum anodes, it is important, because of the high unit cost of platinum to use as small a My platinum electrodes I'l consist of a thin sheet 29 of platinum, about .006 inch thick, which is punched with numerous holes in order to provide ready circulation of the electrolyte through and around all portions of the platinum surface. The thin sheet of platinum is inserted through the cover plate is through an opening l9 provided therein. Above the cover plate, the upper end of the thin platinum sheet is pressed between relatively heavy sheets of copper 25, and the sandwich thus formed is bolted together between the vertical portions of L-shaped bars 26 by bolts 29, which in turn are bolted along the horizontal sides to the cover plates 88 by means of threaded studs 21 of headed machinescrews set in holes in the artificial stone cover plates l8. In order to prevent twisting and buckling of the extremely thin electrode sheet within the cell, it is held in substantially fixed position by means of depending glass bars 28 which are suspended from and rigidly held closely adjacent the platinum sheets 29 in countersunk holes 3!} in the cover plates Hi, the glass bars being provided with flanges 3! to fit in countersunk holes, being cemented in place by suitable chemically resistant material. The glass bars 28 are disposed at intervals along the length of the electrode and extend as far as the depth of the platinum sheet.

The cathode of the cell consists of the surfaces of the steel tank and the steel tubes used for water-cooling which are in contact with the electrolyte. An upright bar member welded to the flange on one side at the top edge of the tank provides a convenient electrical connection for the cathode to the bus bars of the electrical system. The voltage is maintained at about 5 /2 to 6 volts; the anode current density is about amperes per square foot and the cathode current density is about 250 amperes per square foot. The recirculation also has the important function of keeping the electrolyte at a uniform composition, and of uniform temperature of about 55 0., both conditions being important in maintaining the optimum conditions for efficient production of perchlorate. There is alsoconstantly withdrawn from the cell a stream of efliuent electrolyte correspending to the rate of feedof newly dissolved salt solution into the electrolytic cell as influent electrolyte. The withdrawn'electrolyte is accumulated in a storage tank C from which it is withdrawn intermittently to charge the auxiliary electrolytic cell D. This charging liquid to the auxiliarycell, in a typical case, contained about 600 grams per liter of sodium perchlorate and about ,60 grams per liter of sodium chlorate to gether with the sodium chromate and magnesium chloride in the concentrations of the original liquid. The liquid is charged into the auxiliary electrolytic cell in sufiicient amount to fill it, and

it is then electrolyzed between platinum anodes and steel cathodes at a voltage of .5 to 6 volts, the cathode current density being at the beginhim; about 140 amperes persquare foot and-the anode current density being about 250 amperes per square foot, this decreasing as the'conversion to perchlorate becomes completed. The electrolyte in the auxiliary cell D is recirculated to produce a moving electrolyte which assists in the completion of the conversion of the remaining sodium chlorate to sodium perchlorate. As before this recirculation of the electrolyte has the advantage of bringing fresh electrolyte intocontact with the electrodes and in sweeping out the gases which tend to form in layers on the electrode surfaces. The constant recirculation also keeps the electrolyte at a uniform temperature of about 55 G and the-composition of the electrolyte is kept uniform throughout the cell. When the auxiliary electrolyte has been substantially completely'converted to sodium perchlorate, it is withdrawn from the cell into a storage tank. In the typical case which is being described, this liquor contained about 680 grams per liter of sodium perchlorate withlonly about 19 grams per liter of sodium chlorate along with the sodium chromate and magnesiumchloride of the original charge.

The collected effluent electrolyte from .the auxiliary cell is then withdrawn in batches from the storage tank and delivered to a precipitator E where it is treated with sufiicient barium chloride to remove the chromate as insoluble barium chromate and also with sufficient sodium .carbonate to remove the magnesium as insoluble magnesium carbonate. The liquid containing the precipitated barium chromate and magnesium carbonate solid particles is then pumpedinto a I thickener tank G in which the insoluble solids are settled out and withdrawn as a sludge from the bottom of the tank, to be discarded or further treated for recovery of the several ingredients. The clarified liquor from the thickener G is treated in batches in the crystallizer K with a saturated solution of potassium chloride which has previously been dissolved in water in the dissolver J somewhat in excess of the theoretical amount necessary to precipitate out the potassium perchlorate. The crystallizer is provided with means for chilling the liquid in order to precipitate the relatively insoluble potassium perchlorate and after the crystals have attained a sufficient size for good separation, the mixture of crystals and mother liquor is passed through a centrifuge to separat them, the crystals being subsequently dried as substantially pure potassium perchlorate. The mother liquor containing small amounts of sodium and potassium chlorate, perchlorate, and chloride is sent to waste or it may be reclaimed in separate processes.

While I prefer to use the two stage electrolytic process, the eilluent electrolyte from the main cell may be treated in the precipitator F (omitting the auxiliary electrolytic cell), but the purity of the perchlorate salt is not so high, and the efficiency of the whole process is not as satisfactory. One principal advantage of the two stage process is that it permits operation of the main electrolytic step under a good power load factor, while only the auxiliary cell, especially toward the end. of the cycle, need be operated with a poor load factor.

By my process, I am able to produce sodium perchlorate of high purity and with a current efficiency above 9&%. These results are attainable because of the maintenance of optimum uniform conditions on the electrolytic cells. 1. e., the constant recirculation to give a uniform elec- 6 trolyte --composition, temperature, and acidity, and theremovalof gas films from the electrodes; andthe use of the main and the auxiliary electrolytic cell, which permits-operation of the main cell under continuous feed conditions designed to convert most of the chlorate to perchlorate, and the completion of the change to perchlorate in batch cells.

'I claim:

1. In the electrochemical conversion of an aqueous sodium chlorate solution to sodium perchlorate solution, the steps comprising essentially making an aqueous electrolyte consisting essentially of from about 500 grams per liter to a saturatedsolution of sodium chlorate, from 3 to 151 grams of sodium chromate and 1 to 10 grams of magnesium chloride being present per liter 10f "solution as catalysts; and electrolyzing said solution between steel and platinum electrodesunder electrical conditions increasing the perchlorate content while rapidly circulating said electrolyte out-and into the electrolytic cell, and While .maintaining the electrolyte in the cell at a uniform pI-I value in the range of 6.0 to 6.8, and a uniform temperature of about 55 centigrade throughout the body of said electrolyte.

'2. In the electrochemical conversion of an aqueous sodium chlorate solution to sodium perchlorate solution the steps comprising essen- 'tlallly making an aqueous electrolyte consisting essentially of from about 500 grams per liter to a saturated solution of sodium chlorate, from 3 to .15 grams of sodium .chromate and from 1 to .10 grams .of 'magnesium chloride being present per liter of solution as catalysts; maintaining the electrolyte at a uniform pH value in the range from 6.0 to 6.8 and at a uniform temperature of about 55 centigrade; continuously feeding in. a stream of said solution as infiuent electrolyte to an electrolytic cell; rapidly recirculating said electrolyte while electrolyzing the electrolyte between steel and platinum electrodes under electrical conditions increasing the perchlorate content; and continuously withdrawing a portion of said electrolyzed electrolyte as eliluent at a rate corresponding to the rate of feeding in said influent electrolyte.

3. A two-stage electrochemical process for conversion of sodium chlorate to sodium perchlorate, comprising essentially (1) the process of claim 2, in which is produced an eiiluent electrolyte consisting essentially of not less than 500 grams of sodium perchlorate, not more than about 10 percent by weight of sodium chlorate, from 3 to grams of sodium chromate, and from 1 to 10 grams of magnesium chloride per liter; and

(2) the step of electrolyzing as a batch a quantity of said efiluent electrolyte while maintaining a pH in the range from 6.0 to 6.8 and a temperature of about (3., between steel and platinum electrodes at a voltage sufficient to convert chlorate to perchlorate until substantially all of said sodium chlorate is converted to sodium perchlorate.

4. A two-stage electrochemical process for conversion of sodium chlorate to sodium perchlorate, comprising essentially (1) the process of claim 2, in which is produced an eliluent electrolyte consisting essentially of about 600 grams of sodium perchlorate, not more than about grams of sodium chlorate, from 3 to 15 grams of sodium chromate, and from 1 to 10 grams of magnesium chloride per liter; and (2) the step of electrolyzing as a batch a quantity of said effluent electrolyte while maintaining a pH in the I range from 6.0 to 6.8, and a temperature of about 1 55 C;, between steel and platinum electrodes at 1 a voltage in the range from 5 /2 to 6 volts, until substantially all of said sodium chlorate is converted to sodium perchlorate.

5. In the electrochemical conversion of an 1 aqueous sodium chlorate solution to sodium perchlorate solution the steps comprising essentially making an aqueous electrolyte consisting essentially of about 600 grams per liter of sodium chlorate, from 3 to 15 grams of sodium chromate and from 1 to 10 grams of magnesium chloride being present per liter of solution as catalysts; maintaining the said electrolyte at a pI-I value in the range from 6.0 to 6.8 and at a uniform temperature of about 55 centigrade;

I rapidly recirculating the electrolyte through the cell continuously feeding in. a stream of said solution as influent electrolyte to an electrolytic cell; electrolyzing the electrolyte between steel and platinum electrodes under electrical conditions increasing the perchlorate content; and continuously withdrawing a portion of said electrolyte, as eflluent at a rate corresponding to the feeding in of said influent electrolyte.

6. In the electrochemical conversion of an aqueous sodium chlorate solution to a sodium perchlorate solution, the steps comprising essentially making a nearly saturated aqueous electrolyte of sodium chlorate in which is dissolved from 3 to grams per liter of sodium chromate and from 1 to 10 grams per liter of magnesium chloride as catalysts; maintaining said solution at a uniform pH value in the range from 6.0 to 6,8 and at a uniform temperature of about 55 centigrade; and electrolyzing said electrolyte between steel and platinum. electrodes under electrical conditions increasing the perchlorate content while rapidly recirculating the electrolyte through the cell.

JOSEPH C. SCI-IUMACI-IER.

REFERENCES CITED The following references are of record in the OTHER REFERENCES Industrial Electrochemistry, C. L. Mantell, 1st edition, 1931, pages 88-90.

'Mellor, Inorganic and Theoretical Chemistry, vol. 2, 1922, page 395.

Chemical Age, November 13, 1943, pages 493, 494.

Principles of Applied Electrochemistry, Allmand, 1924, pages 389, 488 and 489. 

1. IN THE ELECTROCHEMICAL CONVERSION OF AN AQUEOUS SODIUM CHLORATE SOLUTION TO SODIUM PERCHLORATE SOLUTION, THE STEPS COMPRISING ESSENTIALLY MAKING AN AQUEOUS ELECTROLYTE CONSISTING ESSENTIALLY OF FROM ABOUT 500 GRAMS PER LITER TO A SATURATED SOLUTION OF SODIUM CHLORATE, FROM 3 TO 15 GRAMS OF SODIUM CHROMATE AND 1 TO 10 GRAMS OF MAGNESIUM CHLORIDE BEING PRESENT PER LITER OF SOLUTION AS CATALYSTS; AND ELECTROLYZING SAID SOLUTION BETWEEN STEEL AND PLATINUM ELECTRODES UNDER ELECTRICAL CONDITINS INCREASING THE PERCHLORATE CONTENT WHILE RAPIDLY CIRCULATING SAID ELECROLYTE OUT AND INTO THE ELECTROLYTIC CELL, AND WHILE MAINTAINING THE ELECTROLYTE IN THE CELL AT A UNIFORM PH VALUE IN THE RANGE OF 6.0 TO 6.8 AND A UNIFORM TEMPERATURE OF ABOUT 55* CENTIGRADE THROUGHOUT THE BODY OF SAID ELECTROLYTE. 